Aerosol generator



ug- 10, 1954 R. M. KELL ETAL AEROSOL GENERATOR Filed Nov. 6, 1944 gmc/Moms @P7- M. KELL A ALBERT f? TVQ/1,46

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Patented Aug. 10, 1954 UNITED mais AEROSL GENERATOR Application November 6, 1944, Serial No. 562,242

4 Claims.

The invention described herein may be manufactured and used by or for the Government, for governmental purposes, without the payment to us of any royalty thereon.

lThis invention relates particularly to a method and apparatus for producing a white smoke-like aerosol of an oil.

The production of aerosol fog or smoke in large quantities, known militarily as a screening or blanketing smoke, has the purpose of preventing aimed bombing by enemy aircraft and of hiding ground targets from aerial observation.

A number of different methods and apparatus for forming oil smokes and fogs have been tried. They include mainly: (l) incomplete combustion of a smoke producing oil; (2) vaporizing a fog oil in a boiler by indirect heat exchange; (.3) spraying a fog-producing oil against a heated surface for evaporation; and (4) vaporizing of a fog-producing oil with steam. The rst method was one of the earliest and had the disadvantage of giving a grey or black smoke of low obscuring power with high oil consumption. Of all the prior methods, the one involving vaporization with steam in an apparatus known as a mechanical smoke generator was the most successful. In the mechanical smoke generator, water is pumped through a steam generating coil, a stream of relatively cooler oil is dispersed by an injector into the steam leaving the coil, and the oil-steam mixture is further heated in a coil from which a resulting vapor mixture is emitted through orifices into the air. Although the mechanical generator efiiciently makes a goed smoke, it is somewhat cumbersome with a large amount of machinery requiring attention and experience in operation, is slow in starting, and requires the carrying of large amounts of equipment and material, including a pump capable of delivering liquid at pressures up to 14S lbs. gage and pure soft water.

An object of this invention is to provide a method and apparatus for generating an oil fog or smoke aerosol using hot combustion or iiue gas to vaporize a spray of fog-forming oil with such improvements in design as to make the apparatus light in weight, simple in manufacture and operation, quick in starting and efficient, also, to give other advantages as the need arises.

A characterizing feature of a type of smoke generator designed to accomplish the objects of this invention resides in atomization of a fogproducing oil by gas under pressure, as in an airbrush similar to that used in paint spraying, into the bottom of a vertical vaporization and 2 exhaust duct that is externally heated so that the atomized oil is rapidly vaporized in a steam of hot flue gas passing through the duct. Compressed gas used for the atomization is easily supplied by a small air compressor unit or compressed gas tank and provides the following additional valuable effects: (1) induced draft on the oil burner producing the iiue gas to hasten starting; (2) a required suitable high velocity in the exhaust duct to prevent ignition of the fog oil spray or its vapors and carbonization: (3) suflicient pressure in the exhaust duct to give a high vapor emission velocity from the exhaust duct so as to obtain optimum fog particle size; and (4) improved vaporization temperature control to reduce cracking and decomposition of the fog oil. Besides giving these desired eifects, the use of a compressed gas may be made advantageously with modification of the apparatus for feeding the fog-producing oil, fuel, or any other liquid in the system with the elimination of pumps and other moving me? chanical parts.

A smoke generator designed according to theprinciples of the present invention is practically independent of climatic conditions and may dis; pense with the use of Water, which adds to the transportation problem and gives great difculty of freezing in cold climates.

For showing how the objects and advantages mentioned, also, others which wil become apparent, are attained, the invention will be described in detail with reference to the accompanying drawing, in which is illustrated diagrammatically a flow plan view of one practical embodiment of the aerosol or smoke generator unit.

The aerosol unit illustrated comprises 3 subassemblies: (1) an aerosol generator connected to a fuel oil tank, (2) a fog oil tank, and (3) a gasoline driven air compressor and fog oil pump.

Briefly, the aerosol generator comprises, in general, an outer shell or body 5 having a bot tom closure 6, an inner shell or exhaust duct 'i suspended within and spaced from said outer shell 5, a top closure 8 for the outer shell surrounding said inner shell l', a fuel oil burner 9 disposed near the top of the outer shell 5 for producing hot combustion gases passed in a stream between the inner shell 'l and the outer shell 5 thence exhausted through the inner shell l, and atomizing means lo extending through the bottom closure 6 of the outer shell 5 for centrally spraying a fog-producing oil into the combustion gases as they enter the bottom opening of the inner shell '1.

The outlet El of the inner shell or exhaust tube l is one or more small vent holes or emission slots for discharging fog oil vapors mixed with gases in constricted streams at high velocity with reduction in pressure to atmospheric air surrounding the apparatus.

Primary air is admitted to the combustion zone of burner through an opening l2, and a hole I3 is provided for insertion of a lighting tube or igniter, which may be an electrical igniter or simply a match to start the burning of fuel oil injected from burner nozzle 9. A baiiie wall I d, or similar shield between the burner S and the exhaust tube l', is desirable to prevent direct impingement of hot flame from the burner on the exhaust tube. The baille or shield may completely or partly surround the exhaust tube l to avoid hot spots on the exhaust tube and should be spaced. from the exhaust tube l to provide a passageway for hot flue gas to the annular space between the exhaust tube l and the outer shell 5 down to the inlet at the bottom of the exhaust tube 7 for effecting even heat distribution around the exhaust tube For conservation of heat, the outer shell 5 of the generator may be covered with insulation l5 and with an outer jacket i6.

In order to improve atomization of a normally viscous fog oil and accommodate for various climatic conditions, a preheating coil or heat exchange means l? is located in the lower part of the annular space between outer shell e and the exhaust tube 'i for preheating the fog oil. The fog oil passed through the preheating coil Il abstracts heat by indirect heat exchange from the hot nue gas in the down draft stream after the hot flue gas has imparted some of its heat to contents in the upper part of exhaust tube 'l by indirect heat exchange. The outlet end of preheating coil l? may be connected to atomizer IG by pipe i3 through an automatic, thermostatic, or simply a manually controlled valve i9. A suitable thermostatic Valve for this purpose is responsive to high temperatures, e. g., about 40G-560 F., so that the valve gradually opens With increasing oil temperature to deliver the heated oil under pressure to the atomizer Eil through pipe SS. The valve opening can be limited to a degree at which a good white smoke is produced. Preheated fog oil is thus supplied to the atomizer l0 from supply tank 2li and compressed air or other compressed gas suitably unreactive with the fog oil under conditions existing in the exhaust tube l? is supplied from any available source to atomizer Il! through pipe 2 as from compressed air tank 22.

The fuel oil tank 23 may simply be provided with a cap 2t, an air supply line 25, an oil. line 25 to the burner 9, a safety pressure relief valve 2?, and a clean-out drain plug 23a. For smoother operation, the tank 23 is divided into two compartments by a central partition 28. A substantially constant head of oil is maintained in the lower compartment by manipulating the divider valve 29, which controls the flow from the upper compartment. A series of pet cocks 3U at different levels of the tank may be used for draining and testing for liquid levels. The air inlet line 25 may be provided with a check valve 3l, a pressure gauge 32, the pressure relief valve 2, and the hand valve 34 on the bypass 35 to the upper compartment. The oil is sent from the lower compartment of tank 23 under air or gas pressure to the burner nozzle 9 through pipe Z5 controlled by the outlet valve Et.

Although one oil tank is sucient for supplying an oil to be used both in the burner for making the hot combustion gases and in the atomizer as a fog oil, particularly if the unit is to be made light in weight and is to be used for small scale operations, for units which are to be carried on a truck and used for large scale operations, it is technically advantageous and more economical to have a separate tank for special fog oil with a lower grade fuel oil in a fuel oil tank for burning.

The fog oil tank 26 may simply comprise a filler cap 3l' with a screen 33, a drain valve 39, and an outlet line t3 to the generator.

For controlling the flow of fog oil from tank 2G, the pipe 30 may be provided with a valve M, a pump 42, a pump by-pass 43 with a pressure regulator M, a pump outlet valve 45, and a vent line M5 with valve 47. The supply line All from the fog oil tank passes the fog oil to the heat exchange coil il, from which the heated oil is finally passed to the atomizer li.

A commercial type of gasoline engine driven air compressor may be assembled with the unit for supplying compressed air. As conventionally shown in the drawing, the air compressor comprises a gasoline engine 68 with gasoline supply tank 49, compressor 58, safety pressure relief valve 5i, air tank 22, tank discharge valve 52, and distributing line 53. An air strainer may be y placed in the compressed air supply or distributug line for removing dirt or other contaminants. The distributing line 53 is joined to pipe 25 which conducts compressed air to the fuel oil tank 23 and also is connected to line 2l leading to the atomizer lc. The pump s2 may be driven by the air compressor engine 48.

The atomizer l@ may simply comprise a nozzle with inlets for the fog oil and compressed air or gas and having a needle valve `Ell for adjusting the atomizer. The nozzle is positioned to spray the oil centrally and vertically into the exhaust tube 1.

For operation, compressed air or an equivalent compressed gas, such as, nitrogen or carbon dioxide, is supplied to one or more oil tanks from which fog oil and/or fuel oil are obtained. Air is admitted from injector IG as soon as burner 9 is lit to provide induced draft for the burner. When temperatures are suficiently high for vaporization of fog oil sprayed into the exhaust tube l, valve i9 is gradually opened so that the fog oil preheated to about 40G-500 F. in coil l'l becomes sprayed into the exhaust tube T where the spray becomes mixed with hot flue gas which has passed over the preheating coil l?. The oil iiow through the coil Il is regulated by valves 45 and I9, and the hot oil is supplied to the atomizer simultaneously with compressed air from line 2l. The oil spray is vaporized in the exhaust tube 1, and the oil vapors mixed with the flue gas and air are emitted through discharge opening Il to the atmosphere, where the oil vapors become condensed to produce a white smoke-like cloud.

The exhaust tube 'l is preferably constructed of a suitable heat resisting material, e. g. ceramic or steel alloy, light in weight, and formed to provide a Venturi jet eifect with the atomizer l IJ for increasing the velocity of the gases into the exhaust tube to obtain more favorable particle size, better heat transfer between the atomized oil and hot flue gas, and, also, to make the velocity of the gases greater than the rate of ame propagation so that the burner flame is extinguished where the hot nue gas enters the exhaust -tube l.

The atomizer l in coaxial alignment with the exhaust tube l injects the atomized fog oil centrally through the Venturi throat 55 of the exhaust tube li, and the flue gas is induced to now by the jet effect of the sprayed fog oil and compressed gas into the throat 55. The atomized oil becomes vaporized almost instantaneously after entering the exhaust tube in what may be considered as a vapcrizing zone and does not substantially contact the heated wall of the exhaust tube l'. For satisfactory operation, the temperatures and proportions of the fog oil and compressed air passed from the atomizer and of the flue gas entering the vaporizing zone in exhaust -tube l should be such that the oil is not subjected to decomposition prior to vaporization in the vaporizing zone. There may be some cooling effect when the gases and spray jet through the Venturi .throat 55 becomes rapidly expanded lbut there is not suicient reduction in temperature to prevent rapid vaporization of the atomized oil. The atomizer may be constructed tohave an automatic valve control in conjunction with a thermal element in a preheating coil so that the atomizer operates -to spray the fog oil when the fog oil temperature in the preheating coil H reaches a desired point, such as about 250 C.

lt has been determined that a particle size of about 0.6 micron diameter provides optimum screening power. The approximate particle size of the dispersed oil can be determined by observing the color of the sun viewed through the smoke. Favorable particle size is obtained when the sun color ranges from a deep orange to red.

In producing the oil fogs with the generator of this invention, a jet of oil vapors is discharged into the atmosphere -to undergo rapid condensation. The generator operates at highest eiciency in producing a white smoke-like fog, the eiiiciency being measured by the amount of oil used to substantially obscure an object.

Various fog-producing oils may be used, but in selecting the oil, consideration is given to the availability and the ease of handling, tendencies of the smoke to be toxic or injurious to equipment, and the property of the oil with respect to vaporization, carboniization, pour points, and the stability or persistency of the aerosol dispersion.

Of the oils considered, petroleum oils were selected as the most suitable. `Oleic acid gives a persistent fog but with rancid odor. Fuel oils do not give a very persistent smoke. Petroleum oils having characteristics of light lubricating oils with mid-boiling points of about Z50-800 F. at atmospheric pressure prove satisfactory.

A petroleum oil known as Diol 55 was efliciently used. It is a distilled lubricating oil cut with a minimum flash point of about 385 F., a low pour point, a substantially negligible carbon residue, and a mid-boiling point of about 780 F. Analysis of condensed vapor samples and odor of smoke from this oil indicates that very little cracking takes place in the exhaust tube. The vaporization of the sprayed oil is almost instantaneous, and any small amount of cracking which may take place occurs where the atomized oil strikes the hot side walls of the exhaust tube does not affect the efficiency of the generator. Flue gas analysis shows that very little oxygen in excess air supplied to the burner and in the air used for atomization undergoes combustion in the exhaust tube.

Although the generator operates satisfactorily without the use of water or steam with advantages that are readily appreciated, it may be modified easily to introduce a small amount of steam through an adjustable restriction into the oil fog preheating coil by having a small amount of water fed through a steam coil. By using steam, the vaporization pressure of the fog oil may be reduced, and accordingly, the characteristics of the fog oil may be more varied. A pressure regulator maintained at a set pressure di'lerence across an adjustable valve may be placed in the water feed line to Ithe steam coil so that the water ow to the steam coil is constant regardless of variations in the steam coil pressure.

For the purpose of illustration without limiting the invention, data on a model unit developed in accordance with this invention is given as follows:

1n 59. Fog oil temperature, C 251. Color of sun Orange.

The sub-assemblies were bolted together with shock absorbing rubber pads to reduce vibration. The unit has a `to-tal weight of 1,250 pounds without charge and fits a QM l-ton trailer.

Results from testing the model unit showed that the generator can be started in less than 1 minute. The fuel and fog oil could be fed to the generator with any available compressed gas supply at about 60 lbs. per square inch. The operation of the generator was independent of climatic conditions except that in extremely cold weather, it was desirable to cut the fog oil in viscosity. Ignition of the fog oil vapors in the exhaust tube was avoided.

The described generator unit is adapted to be operated with relatively low tank pressures for now of fog oil, fuel oil, and even water, if desired, and thus may be provided compressed air or gas, such as nitrogen or carbon dioxide, in cylinders to entirely eliminate moving parts such as pumps,

engines, and blowers, or the necessary pressure is readily obtained with a small air compressor unit in the eld. The reduction and elimination of moving parts beneficially eliminates mechanical diiculties and reduces the total weight to be carried. For a portable generator, the liquid and gas supply tanks may be equipped with hand pumps which are capable of producing pressures of the order of 60 pounds per sq. in.

The type of generator described, produces screening smokes which are thick voluminous clouds of high obscuring power. Although the oil aerosol formed is strictly speaking a fog produced by vaporization of an oil and condensation of the vapors into very ne droplets; in military parlance it is known as a smoke. Though relatively high temperatures of the order of cracking temperatures of the fog oils are necessary to vaporize the high boiling temperature range oils which form persistent oil fogs, with the type of apparatus described, the vaporization of the fog oils is obtained Without substantial cracking, even with little or no steam used for lowering the vaporizing temperature.

Organic dyes, particularly those soluble in the fog oils, may be added to the fog oils to obtain a colored smoke. The generator has a high efficiency with respectJ to utilization of heating Value of the fuel oil and with respect to the obscuring power obtained from a given quantity of fog oil.

It is to be understood that, though the invention has been described with reference to certain specific embodiments that other modications come Within the spirit and. scope thereof.

We claim:

1. In an apparatus `for producing an aerosol having a source of vaporizable uid and a source of flue gases, the improvement comprising a vertically disposed, tubular member having a top closure and a bottom closure, an inner tubular member positioned within and spaced from said rst mentioned tubular member, the space between said tubular members being in communication with said source of ue gases, one end of said inner tubular member extending through said top closure, the other end of said inner tubular member having a Venturi-shaped inlet, and a nozzle extending through said bottom closure at a point beneath said inlet and being in communication with said source of vaporizable fluid.

2. In an aerosol generator, the combination comprising an outer tubular member, a closure at each end of said outer tubular member, an inner tubular member positioned within and spaced from said outer tubular member and having one of its ends extending through one end closure and its other end being Venturi-shaped and positioned adjacent to the other end closure, means for supplying Vaporizable uid through 8 said other end closure, and means for supplying Hue gases to the space between said tubular members.

3. In an aerosol generator, the combination comprising an outer tubular member, a closure at each end of said outer tubular member, an inner tubular member positioned Within and spaced from said outer tubular member and having one of its ends extending through one end closure and its other end being positioned adjacent to the other end closure, means for supplying vaporizable fluid through said other end closure, said means for supplying vaporizable fluid including a conduit which is in part positioned Within the space between said tubular members, and means for supplying flue gases to the space between said tubular members.

4. In an aerosol generator, the combination defined in claim 3 wherein said conduit is coiled about said inner tubular member.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,048,997 Collins July 28, 1936 2,070,038 Batt Feb. 9, 1937 2,352,677 Anderson et al. July 4, 1944 2,437,963 Langmuir et al. Mar. 16, 1948 FOREIGN PATENTS Number Country Date 832,933 France July l1, 1938 

1. IN AN APPARATUS FOR PRODUCING AN AEROSOL HAVING A SOURCE OF VAPORIZABLE FLUID AND A SOURCE OF FLUE GASES, THE IMPROVEMENT COMPRISING A VERTICALLY DISPOSED, TUBULAR MEMBER HAVING A TOP CLOSURE AND A BOTTOM CLOSURE, AN INNER TUBULAR MEMBER POSITIONED WITHIN AND SPACED FROM SAID FIRST MENTIONED TUBULAR MEMBER, THE SPACE BETWEEN SAID TUBULAR MEMBERS BEING IN COMMUNICA- 